Apparent anomalies in surface excesses determined from neutron reflection and the Gibbs equation in anionic surfactants with particular reference to perfluorooctanoates at the air/water interface

The surface tension and neutron reflectivity of several perfluorooctanoic acid salts with monovalent metal cations have been investigated over a wide range of surfactant, counterion, and co-ion concentrations. Previous interpretations of similar surface tension data have indicated that there is significant dissociation of the counterions from the surfactant layer, leading to a prefactor less than 2 in the Gibbs equation for the surfactant in the absence of electrolyte, which could be interpreted in terms of a depletion layer of surfactant in the region underneath the surfactant ion monolayer. We show that this result is an artifact resulting from the presence of small amounts of divalent cation impurity. Once the divalent ions are removed, we obtain complete consistency between the neutron and surface tension results using a Gibbs prefactor of 2 with no significant subsurface depletion. It is concluded that contamination of anionic surfactants by divalent ions may be the most common cause of inaccurate determination of adsorption isotherms of these materials at the air/solution interface.